Apparatus for manufacturing a rubber-metal plate composite

ABSTRACT

A vulcanized rubber-metal plate composite is obtained by overlaying a plurality of unvulcanized rubber layers and metal plates alternately and heating thereof by magnetic induction heating. The vulcanized rubber-metal plate composite is obtained by placing a composite of a plurality of unvulcanized rubber layers and metal plates, each being overlaid alternately, into a place affected by an induction coil; heating the metal plates due to eddy currents formed in the metal plates by applying an alternating current to the induction coil; and vulcanizing the unvulcanized rubber layers due to heat conduction from the heated metal plates.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a vulcanized rubber-metal platecomposite which comprises a plurality of rubber layers and metal platesoverlaid alternately, a method and an apparatus for heating anunvulcanized rubber-metal plate composite which is the original state ofthe vulcanized rubber-metal plate composite, and a method and apparatusfor manufacturing the vulcanized rubber-metal plate composite.

2. Description of the Related Art

Vulcanized rubber-metal plate composites have been used in, for example,anti-seismic dampers or rubber bearings. The anti-seismic dampers areplaced on the foundations of structures, e.g. buildings, bridges andmachines to reduce response acceleration to excitation force due toearthquakes and thus to reduce damage to the structures. A typicalanti-seismic damper for buildings has a large size, i.e., a design loadover 500 tons and a diameter of approximately 1 meter.

Such a rubber-metal plate composite is manufactured by bonding aplurality of vulcanized rubber layers and metal plates alternately or byheating an unvulcanized rubber-metal plate composite to a vulcanizationtemperature while compressing it. A method for bonding the vulcanizedrubber layers and the metal plates is disclosed in Japanese PatentPublication No. 59-19018, in which bonding layers are primarily heatedby induction heating of the metal plates. Since this method requires astep for vulcanizing unvulcanized rubber layers one by one and a stepfor applying a bonding agent on the metal plates and overlaying thevulcanized sheets and the metal plates alternately, it is not desirablefor large size composites such as anti-seismic dampers.

It is therefore preferred that a composite comprising unvulcanizedrubber layers and metal plates is heated while being compressed tovulcanize the rubber layers. General methods for heating unvulcanizedrubber include hot-plate pressing and hot pressing. Also, heatingunvulcanized rubber due to heat transfer from a mold heated byelectromagnetic induction is proposed in Japanese Patent Laid-Open No.57-193340.

Since unvulcanized rubber is a heat-insulating material, the heating ofrubber articles by means of an external heat source, e.g. a hot plate ora mold, needs a long time before the interior of the rubber issufficiently heated, resulting in decreased productivity. Such a trendis noticeable in large size anti-seismic dampers. For example, a largeanti-seismic damper for buildings having a design load of approximately500 tons and a diameter of approximately 1 meter requires avulcanization time of 10 to 20 hours. Heating while compressing such alarge composite for a long time also consumes much energy resulting inincreased production costs.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a method for rapidlyheating an unvulcanized rubber-metal plate composite.

It is another object of the present invention to provide a method forheating an unvulcanized rubber-metal plate composite so as not to causetemperature differences in the composite during heating.

In accordance with the present invention, a vulcanized rubber-metalplate composite is obtained by overlaying a plurality of unvulcanizedrubber layers and metal plates alternately and heating thereof bymagnetic induction heating.

Since the metal plate which is a conductive material can be heated byeddy currents, magnetic induction heating can be used in the heatingprocess of the composite.

Examples of metal plates include steel plates, stainless steel plates,aluminum plates, aluminum alloy plates, copper plates and copper alloyplates. A composite using metal plates other than steel plates barelycauses a decrease in design load due to corrosion compared with acomposite using steel plates.

A second aspect of the present invention is a method for heating anunvulcanized rubber-metal plate composite comprising the steps of:placing the composite comprising a plurality of unvulcanized rubberlayers and metal plates, each being overlaid alternately, into a placeaffected by an induction coil; heating the metal plates due to eddycurrents formed in the metal plates by applying an alternating currentto the induction coil; and vulcanizing the unvulcanized rubber layersdue to heat conduction from the heated metal plates.

In the metal plate which is a conductive material, eddy currents aregenerated due to change in magnetic flux. The magnetic flux densityformed by the induction coil increases with the magnetic characteristicsof the metal plate. The magnetic material enhances the magnetic fieldand the eddy currents and thus generate a large amount of heat. Otherheat sources may be used to supply heat. Preferably, the metal plate isa steel sheet in view of strength and material costs.

Both the top and bottom layers of the composite are metal plates. Thesemetal plates have a larger diameter than that of other metal plates inthe composite, because the protruded section of each end metal layer isused as a flange and provided with holes for fixing the compositeproduct to the foundations. In general, the composite is a solidcylindrical column or a cylindrical column having a central cavity. Thecomposite may also be a polygonal column, such as a triangular prismwith or without an inner cavity.

Preferably, the induction coil is arranged so as to generate eddycurrents in the metal plates sandwiched between the vulcanized rubberlayers along the direction perpendicular to the longitudinal directionof the composite in view of heating efficiency.

A preferable method for generating eddy currents in the metal platesalong the direction perpendicular to the longitudinal direction of thecomposite or along the transverse direction of the composite is to placethe composite inside the induction coil. Arrangement of a plurality ofinduction coils on the periphery of the composite may also be used. Theheat applied to the periphery of the plate rapidly transfers to theentire metal plate having high thermal conductivity, and theunvulcanized rubber layers are heated by the metal plate heated on thewhole. Herein the word "periphery" represents the outer periphery of asolid column or the outer and inner peripheries of a column having acentral cavity. One of or both of the outer and inner peripheries may beheated. The inner periphery can be heated by an induction coil insertedinto the central cavity.

In the second aspect, the alternating current applied to the inductioncoil may have a frequency in a range from 1 Hz to 1 kHz.

Heating efficiency decreases as the frequency decreases. An optimumfrequency is determined so that uniform heating and heating efficiencyare compatible. A frequency of less than 1 Hz needs a long time beforethe composite reaches a given temperature.

On the other hand, a frequency of not greater than 1 kHz does not causetemperature differences between the center and the top or bottom end ofthe composite. The higher the frequency, the more the top and bottomends of the composite are intensively heated. Application of analternating current with a low frequency can generate a uniform magneticfield over all the metal plates of the composite and thus uniformly heatthese metal plates in a short time.

In the second aspect, the frequency of the alternating current appliedto the induction coil may be varied during heating.

Varying the frequency controls heat generation at the periphery and theinside of each metal plate and thus reduces the temperature differencesbetween them.

In the second aspect, the method may be applied to a preliminary stepfor heating the unvulcanized rubber in the composite to a predeterminedtemperature.

The vulcanization of the unvulcanized rubber requires a preliminaryheating step and a vulcanizing step of the preheated rubber at a giventemperature while compressing the preheated rubber. After the preheatingstep is performed by induction heating, the rubber may be vulcanized bya conventional method, such as hot-plate pressing or hot pressing, inthe vulcanizing step.

In the second aspect, the method may be applied to a vulcanizing stepfor heating the composite while pressing the composite in thelongitudinal direction.

After the preheating step is performed by a conventional process, e.g.hot-plate pressing or hot pressing, the rubber may be vulcanized bymagnetic induction heating in the vulcanizing step. The magneticinduction heating can be used in both steps.

In the second aspect, the periphery of the composite may be confined toa mold during heating.

The unvulcanized rubber-metal plate composite is loaded into acompression mold in the vulcanization step. Since the composite isheated in the same mold through the preheating step and thevulcanization step in-situ, loading into and unloading from the mold arenot required between these steps.

In the second aspect, the mold may comprise a nonmagnetic material or aweakly magnetic material.

Since the nonmagnetic or weakly magnetic mold attracts the magneticflux, a satisfactory magnetic field is formed in the hollow cylindricalmold. The magnetic flux formed by the induction coil therefore passesthrough the metal plates of the composite contained in the hollow moldwithout being reduced by the mold, resulting in effective magneticinduction heating.

Although it is preferable that the mold comprise a nonmagnetic material,a weakly magnetic material also has a similar effect. Examples of weaklymagnetic materials include austenitic stainless steels such as SUS304,concrete and ceramics.

In the second aspect, the mold may be provided with a heating means sothat the composite is heated through the periphery due to heatconduction from the mold.

Conventional means of heating, for example, supplying a heating mediumin the cavity formed in the mold, or providing a heating wire in themold can be used in the present invention.

Such a means of heating can offset the temperature differences betweenthe core and the periphery of the composite, i.e., temperature gradientin the transverse direction.

In the second aspect, the method may be performed while heating thecomposite with a means of heating that heats the composite through thetop and bottom ends of the composite and that maintains the temperatureof the composite.

A conventional means of heating, e.g. a hot plate with a circulatingheating medium or with a heating wire, can be used as the means ofheating. The means of heating can offset the longitudinal temperaturegradient.

A third aspect of the present invention is a method for manufacturing avulcanized rubber-metal plate composite described above, wherein thecomposite comprising a plurality of unvulcanized rubber layers and metalplates, each being overlaid alternately, is arranged in a place affectedby an induction coil, an alternating current is applied to the inductioncoil to generate eddy currents in each of the metal plates and to heatthe metal plates, and the unvulcanized rubber layers are vulcanized bybeing heated by means of heat conduction from the heated metal plateswhile compressing the composite in the longitudinal direction.

In this aspect, induction heating is applied at both the preliminaryheating step and the vulcanizing step in the production of a vulcanizedrubber-metal plate composite. Both the steps can be performed in thesame apparatus with high efficiency.

In the third aspect, the composite may be heated in a mold bounding theperiphery of the composite.

Since the composite can be heated in the mold in situ, loading into andunloading from the mold are not required and thus the productionprocesses can be simplified.

A fourth aspect of the present invention is a method for manufacturing avulcanized rubber-metal plate composite described above, wherein thecomposite comprising a plurality of unvulcanized rubber layers and metalplates, each being overlaid alternately, is arranged in a place affectedby an induction coil, an alternating current is applied to the inductioncoil to generate eddy currents in each of the metal plates and topreheat the metal plates, and the unvulcanized rubber layers arevulcanized by heating while compressing the composite in thelongitudinal direction.

The vulcanization of the unvulcanized rubber requires a preliminaryheating step and a vulcanizing step of the preheated rubber at a giventemperature while compressing the preheated rubber. After the preheatingstep is performed by induction heating, the rubber may be vulcanized bya conventional method, such as hot-plate pressing or hot pressing, inthe vulcanizing step.

In the fourth aspect, the composite may be heated in a mold bounding theperiphery of the composite.

Since the composite can be heated in the mold in situ, loading into andunloading from the mold are not required and thus the productionprocesses can be simplified.

A fifth aspect of the present invention is a method for manufacturing avulcanized rubber-metal plate composite comprising the steps of:exposing at least a part of the metal plate among all the metal platesas constituents of the rubber-metal composite from the unvulcanizedrubber layers and heating thereof by induction heating to vulcanize therubber, and forcibly cooling the exposed section of the metal plate.

Force-cooling of the exposed section of the metal plate permits rapidcooling from the inside of the composite due to heat dissipation throughthe metal plate, resulting in a shortened cooling period of thecomposite after vulcanization.

Preferably, the exposed section of the metal plate is forcibly cooledwith a liquid or gaseous cooling medium. Such a cooling medium is easyto handle and prompts the cooling.

In the fifth aspect, the exposed section of the metal plate may beforcibly cooled through a cooling fin attached to the protruded section.

The exposed section can be more rapidly cooled through the cooling finwhich is cooled with a cooling medium.

A sixth aspect of the present invention is an apparatus for heating arubber-metal plate composite comprising a plurality of unvulcanizedrubber layers and metal plates, each being overlaid alternately, byinduction heating, the apparatus comprising:

an induction coil for applying a magnetic field to the composite andheating the metal plates due to eddy currents generated by the magneticfield; and

a power unit for applying an alternating current to the induction coilto generate the magnetic field.

Since the metal plates themselves in the composite are heated by thisheating apparatus, the interior of the composite is directly heatedthrough the metal plates. The entire composite can be more rapidlyheated to a predetermined temperature compared with external heatingapparatuses.

In the sixth aspect, the apparatus may further comprise a mold toconfine the periphery of the composite.

The transition from the preheating step to the vulcanizing step can beperformed smoothly and thus the vulcanizing system can be simplified.

In the sixth aspect, the apparatus may further comprise a heating unitfor heating the mold.

When the mold is made of nonmagnetic or weakly magnetic stainless steel,the mold itself functions as the heating unit by eddy currents generatedin the conductive mold whereas the magnetic flux permeates the moldwithout loss and reaches the composite.

When the stainless steel is not used, the composite can be heated fromits periphery through the mold which is heated by the heating unit. Thetemperature differences between the core and the periphery of thecomposite can therefore be offset, resulting in rapid and uniformheating of the composite.

A conventional heating unit, e.g. a hot plate with a circulating heatingmedium or with a heating wire, can be used as the heating unit.

The composite may be heated by a heating member which comes into contactwith the periphery of the composite and includes a heating unit, insteadof the mold.

In the sixth aspect, the apparatus may further comprise hot plates forheating both the top and bottom ends of the composite.

These hot plates can also be heated by a conventional process, e.g. aheating medium process or an electrical heating process, as in the moldheating process.

The hot plates can offset longitudinal temperature differences and thuspermit rapid and uniform heating of the composite. When the composite isheated by a compression press, the upper and lower platens may be usedas the hot plates.

In the sixth aspect, the power unit may further comprise a frequencymeter for changing and controlling the frequency of the alternatingcurrent.

An optimum frequency of the magnetic field can be applied to thecomposite in response to the size of the composite. Also, the frequencymeter can vary the heat generated in the preliminary heating step andthe vulcanizing step. Accordingly, the frequency meter can adjust thequantity of heating in view of the purpose and permits rapid and uniformheating of the composite.

A seventh aspect of the present invention is an apparatus formanufacturing a vulcanized rubber-metal plate composite comprising anapparatus for heating as described above and a compressing means forcompressing the composite in the longitudinal direction.

The entire composite can be heated and vulcanized in a short time with asimplified apparatus.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a longitudinal cross-sectional view of an embodiment of arubber-metal plate composite in accordance with the present invention;

FIG. 2 is a longitudinal cross-sectional view of another embodiment of arubber-metal plate composite in accordance with the present invention;

FIG. 3 is a longitudinal cross-sectional view of a further embodiment ofa rubber-metal plate composite in accordance with the present invention;

FIG. 4 is a cross-sectional view of an apparatus for carrying out themethod in accordance with the present invention;

FIG. 5 is a graph of heating performance using different heatingsources;

FIG. 6 is a graph for comparing heat-up curves according to a method ofthe present invention with heat-up curves according to a conventionalmethod;

FIG. 7 is a graph illustrating correlations between the frequency andthe heating time and between the frequency and the in-plain temperaturedifference of a steel plate;

FIG. 8 is a longitudinal cross-sectional view illustrating a method forheating a rubber-metal plate composite in a mold in accordance with thepresent invention;

FIG. 9 is a graph illustrating correlations between the frequency andthe heating rate of a stainless steel mold and the stainless steelitself;

FIG. 10 is a block diagram of a vulcanizing system using a method forheating in accordance with the present invention in a preheating step;

FIG. 11 is a longitudinal cross-sectional view of an apparatus forpreheating a rubber-metal plate composite in a mold by magneticinduction;

FIG. 12 is a longitudinal cross-sectional view of an embodiment of avulcanizing apparatus used after a composite is preheated by magneticinduction;

FIG. 13 is a longitudinal cross-sectional view of an apparatuspreheating and vulcanizing a composite;

FIG. 14 is a longitudinal cross-sectional view of an apparatus heating ahollow cylindrical composite by magnetic induction; and

FIG. 15 is a longitudinal cross-sectional view of an apparatus heating ahollow cylindrical composite in a mold by magnetic induction; and

FIG. 16 is a longitudinal cross-sectional view illustrating anembodiment of a cooling step in accordance with the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Embodiments of the present invention will now be described withreference to the drawings. FIGS. 1 to 3 are longitudinal cross-sectionalviews of embodiments of a composite in accordance with the presentinvention. In general, the composite is a solid cylindrical column or acylindrical column having a central cavity. The composite may also be apolygonal column, such as a triangular prism with or without an innercavity.

In FIG. 1, the composite 1 comprises a plurality of metal plates 2, 4and 5 and rubber layers 3, which are overlaid alternately, and thesemetal plates are arranged parallel to each other. The metal plates 2 arealso referred to as inner plates 2, and the metal plates 4 and 5 arealso referred to as connection plates 4 and 5. Both surfaces of eachinner plate 2 come into contact with the rubber layers 3. The connectionplates 4 and 5 are arranged on the top and bottom ends of the composite1 and are provided with flanges 4a and 5a. Each of the inner plates 2and the rubber layers 3 has a thickness in a range of severalmillimeters. The number of inner plates 2 and rubber sheet layers 3which are overlaid alternately ranges from about half a dozen to severaldozens. The connection plates 4 and 5 function as flanges to fix thecomposite to a foundation or a structure. These connection plates 4 and5 are therefore thicker and wider than the inner plates 2. The innerplates 2 are provided to increase the rigidity of the composite in thelongitudinal direction or overlaying direction. It is thereforepreferred that the thickness of each inner plate 2 be almost equal tothat of each rubber layer 3.

In FIG. 1, the peripheries of the inner plates 2 are surrounded by acylindrical rubber layer 3a, and thus each inner plate 2 is completelyembedded in the rubber layers 3 and the cylindrical rubber layer 3a. Theinner plates 3 can therefore be prevented from corrosion in avulcanizing step of the unvulcanized composite.

In FIG. 2, the peripheries of the inner plates 2 are exposed since thecylindrical rubber layer is omitted. In FIG. 3, the peripheries 2a ofsome inner plates 2 are exposed and the other inner plates 2 aresurrounded by rubber. When a mold is attached along the exposedperipheries 2a, the inner plates 2 are heated by thermal conduction fromthe heated mold and the unvulcanized rubber layers 3 can be moreeffectively vulcanized by thermal conduction from the inner plates 2.When the composite is cooled, forced cooling of the peripheries 2a ofthe inner plates 2 with a cooling medium can shorten the cooling time.

The metal plates 2, 4 and 5 are made of an electrically conductivematerial, eddy currents are generated inside the metal plates under avarying magnetic-flux environment, and the metal plates are heated bythe eddy currents. The composite 1 is therefore heated through the metalplates 2, 4 and 5 by magnetic induction heating. When the metal plates2, 4 and 5, which are provided in the path of the magnetic flux, havemagnetic characteristics, the magnetic field is enhanced and thus thequantity of heat increases as eddy currents increase. The rubber layers3 are heated by thermal conduction through the heated metal plates 2, 4and 5. Since the composite 1 is generally used as an anti-seismic damperbelow a column of a structure such as a building, it must be durable fora long time, such as several decades, under a high load, e.g. severalhundred tons. It is therefore preferred that these metal plates be madeof a material having higher corrosion resistance than steel. Examples ofanticorrosion materials include stainless steel, aluminum, aluminumalloys, copper and copper alloys. Use of these anticorrosion materialscan prevent a decrease in design load of the composite due to corrosion.Another preferred material for the metal plates 2, 4 and 5 from thepoint of view of its strength and cost of materials is a martensiticstainless steel.

A method for heating the composite will now be described with referenceto FIG. 4.

In FIG. 4, a spiral induction coil 6 is wound onto the periphery of thecomposite 1 and an alternating current is applied to the induction coil6 through a frequency meter 7. The frequency meter 7 comprises, forexample, an inverter damper and generates an alternating current flowwith an appropriate frequency of 1 kHz or less. Preferably, theinduction coil 6 is arranged between the upper and lower connectionplates 4 and 6 so as to be close to the edge or periphery 2a of theinner plates 2 as much as possible. The magnetic flux generated by theinduction coil 6 forms a loop, as shown in broken lines in the drawing,which passes through the lower surface layer of the upper connectionplate 4, pierces the inner plates 2 near their peripheries 2a and passesthrough the upper surface layer of the lower connection plate 5. Eddycurrents are generated near the peripheries 2a of the inner plates 2,and the inner plates 2 are heated near their peripheries 2a by means ofI² R loss due to the eddy currents.

The inner plates 2 are made of a highly thermal conductive material,whereas the unvulcanized rubber layers 3 being into contact with theinner plates 2 have heat insulating characteristics. The heat generatednear the periphery 2a of each inner plate 2 rapidly dissipates towardthe center of the inner plate 2 as shown by arrow i in the drawing, nottoward the rubber near the periphery. As a result, the inner plate 2immediately has a uniform temperature distribution. The heat accumulatedin the entire inner plate 2 gradually dissipates toward the unvulcanizedrubber layer 3 as shown by arrow ii in the drawing. Since such heatdissipation occurs in all the inner plates 2, the composite 1 as a wholeis rapidly and uniformly heated from its interior.

The heating method by magnetic induction in accordance with the presentinvention will now be compared with a conventional steam heating methodwith reference to the graphs illustrating the results of the simulation.

In FIG. 5, a composite 1 is heated by induction heating to 100° C. byapplying a 60 Hz alternating current or by external steam heating in asteam room. In the legend of the drawing, the word "center" representsposition A in the rubber layer of the composite 1 shown in FIG. 4 and isjust the center in the longitudinal and transverse (or radial)directions of the composite 1. The word "top" represents position B inthe rubber layer of the composite shown in FIG. 4 and is located nearthe periphery 2a of the metal plate 2 and at the top rubber layer.

In FIG. 5, both the center () and the top (▪) are rapidly heated in ashort time by induction heating, whereas the top (♦) and the center (▴)are gradually heated by steam heating. The temperature differencebetween the center and the top is relatively small in the inductionheating as shown by similar heat-up curves in the drawing, whereas alarge difference between the center and the top in the heat-up curves ofthe steam heating demonstrates nonuniform heating of the composite.

In the magnetic induction heating, since the composite is heated fromits interior, it is uniformly heated in a short time and both the centerA and the top B reach a saturated temperature of 100° C. afterapproximately 100 minutes. In contrast, in the steam heating thecomposite is heated from the exterior toward the interior, a significanttemperature difference between the center A and the top B is observedfor a long time, and both the center A and the top B reach saturatedtemperatures of near 100° C. after approximately 20 hours. Accordingly,the results of the simulation suggest that the induction heating inaccordance with this embodiment of the composite 1 shortens thevulcanizing time to one-tenth of that in the steam heating.

A martensitic stainless steel used as metal plates 2, 4 and 5 will nowbe compared with a steel sheet.

FIG. 6 is a graph illustrating the results (∘) of the simulation inwhich a martensitic stainless steel (SUS) sheet having high corrosionresistance is used as the inner plates 2 and the connection plates 4 and5 of the composite 1 and heated to 150° C. by applying a 60 Hzalternating current. FIG. 6 also shows the results of a steel sheet ()having low corrosion resistance and a metal sheet (Δ) heated by aconventional steam heating process at 170° C.

The graph demonstrates that the time required for the plate to reach150° C. from the start is 440 minutes for the induction heating of thestainless steel (SUS) composite, 220 minutes for the induction heatingof the steel composite and 520 minutes for the steam heating. Comparingthe maximum and minimum temperatures, the induction heating of the steelcomposite has the smallest temperature difference therebetween, thesteam heating of the metal sheet has the largest temperature difference,and the induction heating of the stainless steel composite has anintermediate temperature difference. Although the stainless steelcomposite needs a longer induction heating time than the steel compositedue to lower electrical conductivity, the stainless steel composite canbe more rapidly heated by the internal induction heating than theexternal steam heating.

Although the results shown in FIGS. 5 and 6 suggest that inductionheating is superior to steam heating, a slight temperature difference isobserved between the center A and the top B in the composite 1. Morespecifically, FIG. 5 demonstrates that the heat-up curve in the center A() is steeper than that in the top B (▪) and thus the center A isheated slightly more rapidly than the top B in the composite 1. Theseresults suggest that heat dissipation from the peripheries 2a of theinner plates 2 toward the center is superior to that from the top andbottom ends of the composite 1, and the dissipated heat is accumulatedin the center.

It is therefore preferred that the frequency of the induction coil 6 bevaried to control heat formation such that both the top and the bottomhave the same heat-up curves.

On the other hand, an excessively increased frequency of the alternatingcurrent flow by the frequency meter tends to heat intensively theperipheries C and D (refer to FIG. 4) of the highest and lowest innerplates among the inner plates 2 and thus causes temperature gradients inthe longitudinal and radial directions of the composite 1. The frequencytherefore must be 1 kHz or less in order to heat the peripheries 2a ofall the inner plates 2.

Even at a frequency of 1 kHz or less, temperature gradients in theradial direction may form in the metal plates. In this case, applicationof a lower frequency increases the depth of penetration of the magneticflux in the radial direction from the edge of each metal plate. Thejoule heat distribution in the radial direction of the metal platestherefore broadens to level the temperature difference as the frequencydecreases.

FIG. 7 is a graph illustrating the correlations between the frequency ofthe coil and the heating time and between the frequency and thetemperature difference, in which an alternating magnetic field isapplied to a composite with a diameter of 500 mm made by overlaying 25steel sheets with a thickness of 3 mm and 26 vulcanized rubber sheetwith a thickness of 5 mm alternately.

FIG. 7 demonstrates that the in-plain temperature difference (▴) of theinner plate 2 decreases as the frequency decreases, in other words, thecomposite 1 can be more uniformly heated by a lower frequency. On theother hand, the time () required for the entire composite 1 to reach150° C. increases as the frequency decreases. An optimized frequencyrange can therefore be determined using FIG. 7 so that the in-plaintemperature difference does not affect the vulcanization of the rubberand the temperature of the composite rapidly increases. Accordingly, itis preferable that the frequency be in a range from 1 kHz to 20 kHz. Insuch a frequency range, the in-plain temperature difference can becontrolled to within 50 to 70° C., and the time required to reach 150°C. in a range from 10 to 100 minutes. As described above, thetemperature difference in the radial direction of the composite can alsobe reduced by decreasing the frequency of the alternating magneticfield.

Another method for reducing the in-plain temperature difference is tocontrol the temperature of the mold which comes into contact with theperiphery of the composite. In this method, it is preferred that themold be made of an electrically conductive material and be heated bymagnetic induction together with the inner plates. In accordance withthis method, since the composite is also heated through the peripherydue to heat conduction from the mold, the temperature differencesbetween the core and the periphery of the composite can be reduced moreeffectively.

A mold 8 shown in FIG. 8 has a cavity having an inner diameter which issubstantially equal to the outer diameter of the composite 1 and boundsthe composite 1 along the radial direction. The distance between steps8a and 8b of the mold 8 is shorter by a clearance ε for compression thanthe distance between the inside faces of the connection plates 4 and 5of the composite 1. When the composite 1 is heated in the mold 8, theinduction coil 6 is provided around the outer periphery of the mold 8.

FIG. 9 is a graph showing the results of the simulation of thecorrelations between the heating rate and the frequency of an austeniticstainless steel mold (), as shown in FIG. 8, having a diameter of 25 mmand the stainless steel sheet itself (∘). The heating rate of the moldsignificantly increases with the frequency, whereas the heating rate ofthe stainless steel sheet does not change substantially. The two heatingrates agree at a frequency of approximately 3 Hz. Consequently, in theaustenitic stainless steel mold, a frequency of the induction heating ofapproximately 3 Hz does not cause temperature differences between thecore and the periphery of the mold. Further, the heating rates of themold and the inner plates can be independently controlled by varying thefrequency during the heating step in view of the correlations shown inFIG. 9.

It is preferred that the mold 8 be made of a nonmagnetic or weaklymagnetic material, because the magnetic flux from the induction coil 6permeates the mold 8 and reaches the inner plates 2 without attenuation.The magnetic field in the composite 1 therefore is not affected by themold. A typical example of the nonmagnetic or weakly magnetic materialis the above-mentioned austenitic stainless steel. Concrete and ceramicsalso have the same effect.

As described above, the heating of the mold being in contact with thecomposite can decrease the temperature differences. When heating orpreheating the composite not using a mold, an austenitic stainless steelmember may be in contact with the periphery of the composite. Auxiliaryuse of another heating method at a position having a low heating ratecan also offset the temperature differences.

FIG. 10 is a block diagram illustrating an embodiment of a vulcanizingsystem of the unvulcanized rubber-metal plate composite. In thisvulcanizing system, the magnetic induction heating in accordance withthe present invention is used in preheating steps (first and secondpreheating stations S2 and S3), and a conventional steam heating is usedin the vulcanizing step (compression heating station S1). Thevulcanizing system further includes first and second mold assembly &disassembly stations S4 and S5 (or an assembly station S4 and adisassembly station S5), and a conveyor bogie 30 which carries the moldand the composite between these stations S1 to S5.

Although the composite can be heated without a mold in general, thevulcanizing step with compression requires heating in a compressionmold. It is therefore preferred that the composite be vulcanized in themold on an assembly line from the preheating step to the vulcanizingstep. The composite, in FIG. 10, is transferred to the mold assemblystation S4 to assemble the mold, preheated at the preheating station S2or S3, vulcanized at the compression heating station S1 and transferredto the mold disassembly station S5 to detach the mold.

In general, the preheating step for heating the composite to a giventemperature requires a longer time than the vulcanizing step forcompressing the composite while maintaining the given temperature. Theassembly and disassembly of the mold also requires an additional amountof time. The number of the preheating stations is therefore set to begreater than the number of the vulcanizing stations in order to shortenthe vulcanizing cycle and improve the productivity of the composite.Similarly, it is preferred that a plurality of mold assembly &disassembly stations be provided. The vulcanizing system in FIG. 10 hastwo preheating stations and one compression heating station. The ratiosbetween these numbers are determined in view of time periods requiredfor these steps.

Apparatuses shown in FIGS. 11 and 12 are used in the system shown inFIG. 10. Specifically, the preheating step is performed by magneticinduction using the apparatus shown in FIG. 11 and the vulcanizing stepis performed by steam heating using the apparatus shown in FIG. 12.

In FIG. 11, a mold 10 includes a cylindrical mold main body 12 boundingthe periphery of the composite 1, and a lower cover 11 and an uppercover 13 which come into contact with the bottom and top ends of thecomposite, respectively. The lower connection plate 5 of the compositeis supported by the lower cover 11 and the mold main body 12 and theupper connection plate 4 is supported by the upper cover 13 and the moldmain body 12. A clearance ε for compression is provided between theupper cover 13 and the mold main body 12. The induction coil 6 isprovided on the outer periphery of the mold main body 12 within a rangebetween the connection plates 4 and 5. A heating jacket 12a used in thevulcanizing step is provided inside the mold main body 12. Use of avertically dividable mold main body 12 helps loading and unloading ofthe composite 1 into the mold. Dividable upper and lower covers are alsouseful. The upper cover 13 and/or the lower cover 11 may be omitted.

The mold main body 12 is made of, preferably, a nonmagnetic or weaklymagnetic material, such as SUS304, as described above so that themagnetic flux from the induction coil permeates the mold 10 and reachesthe inner plates 2. When a low frequency alternating current is appliedto the induction coil 6, the entire composite 1 is rapidly heated fromits interior as the preheating step. Since SUS304 is an electricallyconductive material and thus is heated by magnetic induction, thecomposite 1 is also heated through the periphery being in contact withthe mold main body 12 so as not to form temperature differences betweenthe core and the periphery.

In FIG. 12, a vulcanizing press 20 comprises a press frame 21, an upperplaten 22 provided under the top face of the press frame 21 through aheight adjusting mechanism 23, and a lower platen 24 provided on thebottom face of the press frame 21 through a pressure cylinder 25. Aftershrinking the pressure cylinder 25, the mold 10 containing the preheatedcomposite 1 is loaded on a given position in the vulcanizing press 20by, for example, sliding the mold 10 in the transverse direction. Next,the pressure cylinder 25 is expanded to compress the composite 1 in themold 10 through the upper and lower platens 22 and 24, and steam issimultaneously introduced into the heating jacket 12a in the main body12 to heat and vulcanize the composite 1. Heating jackets 22a and 24a tointroduce steam may be provided inside the upper and lower platens 22and 24.

The magnetic induction heating in accordance with the present inventionmay be used in the vulcanizing step. In this case, it is preferred thatthe preheating and vulcanizing steps be performed in the same apparatusas shown in FIG. 13. Alternatively, a combination of the preheating stepby a conventional heating process such as steam heating and thevulcanizing step by the magnetic induction heating may be also usable.

A heating apparatus which performs preheating and vulcanization will nowbe described in detail with reference to FIG. 13.

The heating apparatus includes a press 40 vertically pressing thecomposite 1 loaded into the mold 43 and a magnetic induction heatingunit 41 heating the composite 1. The press 40 is provided with a hollowframe 31. The hollow frame 31 is formed by stacking a plurality of plateframe pieces 32 and fixing them to each other with bolts 33. The numberof plate frame pieces 32 varies with the applied stress and the formedstrain.

A spacer 34 hangs from the upper side of the frame 31, and is providedwith an upper platen 36 with an inner cavity 36a through a platensupporter 35 on the bottom end of the spacer 34. A lower platen 37 withan inner cavity 37a is arranged below the upper platen 36. The innercavities 36a and 37a are connected to a heat source not shown in thedrawing. The upper and lower platens 36 and 37 are heated to a giventemperature or higher by a heating medium, such as steam, supplied tothe inner cavities 36a and 37a through the heat source.

The lower platen 37 is supported on a hydraulic pressure cylinder 39through a platen supporter 38. The hydraulic pressure cylinder 39 isprovided with a pressure rod 39a having a vertical axis, and the lowerplaten 37 lifts or lowers in response to the movement of the pressurerod 39a. The mold 43 containing the composite 1 is placed between theupper and lower platens 36 and 37. The mold 43 has a hollow cylindricalshape so as to come into contact with the composite 1 and has a cavity43a which is connected to a heat source not shown in the drawing duringthe vulcanizing step. The mold 43 is heated to a given temperature orhigher by a heating medium, e.g. steam, supplied to the cavity 43athrough the heat source. The mold 43 is made of a nonmagnetic material,such as SUS304, to satisfactorily form a magnetic field in the mold.

The composite 1 loaded into the mold 43 has a cylindrical unvulcanizedrubber 3. A plurality of metallic inner plates 2 are arranged at a giveninterval in the unvulcanized rubber 3. Metallic connection plates 4 and5 are joined to the top and bottom ends of the unvulcanized rubber 3.These connection plates 4 and 5 and inner plates 2 heat the unvulcanizedrubber 3 from both ends and the interior by magnetic induction heating.

The connection plates 4 and 5 and the inner plates 2 are heated by amagnetic induction heating apparatus 41 arranged on the periphery of themold as shown in FIG. 13. The magnetic induction heating apparatus 41 isprovided with a circular induction coil 47 and a cooling unit whichforcibly cools the induction coil 47 with air or water. The inductioncoil 47 is connected to a frequency meter 7 (power unit) shown in FIG. 4and the frequency meter 7 generates a magnetic field having a givenintensity around the induction coil 47 while varying the frequency ofthe alternating current flow applied to the induction coil 47.

The operation of the heating apparatus will be described.

The mold 43 and the induction coil 47 are loaded into the press 40, andthe pressure rod 39a is extended from the pressure cylinder 39 tocompress the composite 1 in the mold 43 through the upper and lowerplatens 36 and 37. A heating medium, e.g. steam, is supplied to thelower and upper platens 36 and 37 and the mold 43 to heat them, while analternating current having a given frequency is applied to the inductioncoil 47 to generate a magnetic field around the induction coil 47.

The magnetic field permeates the upper and lower connection plates 4 and5 being in contact with the unvulcanized rubber and the inner plates 2embedded into the unvulcanized rubber 3 and generates eddy currents inthese plates. The unvulcanized rubber 3 is heated at the inside and theperiphery by the heat due to eddy currents from the connection plates 4and 5 and the inner plates 2. The unvulcanized rubber 3 is vulcanized bythe continuation of such heating. After the vulcanization of thecomposite 1 is completed, the mold 43 containing the vulcanizedcomposite 1 is transferred from the press 40 to the subsequent step,such as a cooling step.

Any other heating means, e.g. electric heating, may be used for heatingthe upper and lower platens 36 and 37 and the mold 43 instead of heatingby a heating medium such as steam. When a temperature difference occursduring the induction heating, it is preferable that the region having alower temperature be additionally heated by these heating means in viewof rapid and uniform heating.

In the above embodiment, the induction coil is spirally wound along theperiphery of the composite 1. Alternatively, a plurality of bellowinduction coils may be provided on the outer periphery of the composite1, or the composite may be arranged so as to be affected by the magneticflux from a U-shaped iron core with a wound coil. Consequently, it isimportant that the magnetic flux permeates the inner plates in thecomposite 1 and more specifically that it heats the radial edges of theinner plates.

The composite 1 may have a cavity. A method for heating a hollowcylindrical composite 50 will be described with reference to FIGS. 14and 15. The hollow composite 50 shown in FIG. 14 made by embedding innerplates 52, having center holes, into unvulcanized rubber 53 and adheringthe connection plates 54 and 55, having the same diameter as that of theunvulcanized rubber 53, onto the upper and bottom end of theunvulcanized rubber 53. In such a composite configuration, inductioncoils 61 and 62 are arranged on the inner and outer peripheries of thecomposite 51 to heat the composite from the two peripheries.

In FIG. 15, the composite 51 is loaded into a mold 63 to be heated bymagnetic induction through the mold. The mold 63 comprises a lower moldsection 66 having a vertical outer cylinder 64 and a vertical centralcolumn 65 and an upper mold section 67 which covers the lower mold 66 soas to form a clearance ε for compression. The mold 63 is made of anonmagnetic material, such as SUS304, and heats the composite 51 by aninduction coil 6 arranged on the outer periphery of the outer cylinder64 of the lower mold section 66. The outer cylinder 64 and the centralcolumn 65 have heating jackets 64a and 65a, respectively therein toadditionally heat the composite 1 by, for example, steam heating.

The composite 1, in which the inner plates 2 are exposed as shown inFIGS. 2 and 4, can be cooled from the inside by heat conduction throughthe inner plates 2 which are forcibly cooled at their exposed edges in acooling step after the vulcanization. The cooling time can therefore bereduced.

The composite may be cooled by spontaneous heat dissipation or by forcedcooling as shown in FIG. 16 in which a fan 14 blows air as a coolantonto the exposed edges of the inner plates 2. Gaseous nitrogen may beused instead of the air, or a liquid coolant, such as water or oil, maybe used instead of the gaseous coolant. Alternatively, cooling fins 15may be provided on the exposed sections to promote the spontaneous heatdissipation. A combination of the cooling fins 15 and a gaseous orliquid coolant will further accelerate the cooling of the composite 1.When using air or water as a coolant, it is preferred that the innerplates 2 be made of a stainless steel sheet in order to preventcorrosion of the inner plates 2 due to the coolant.

What is claimed is:
 1. An apparatus for heating a rubber-metal platecomposite comprising a plurality of unvulcanized rubber layers and metalplates, each being overlaid alternately, by induction heating, saidapparatus comprising:an induction coil for applying a magnetic field tosaid composite and heating said metal plates due to eddy currentsgenerated by the magnetic field; a power unit for applying analternating current to said induction coil to generate said magneticfield; a mold to confine the periphery of said composite; and a heatingunit for heating said mold.
 2. An apparatus for heating a rubber-metalplate composite comprising a plurality of unvulcanized rubber layers andmetal plates, each being overlaid alternately, by induction heating,said apparatus comprising:an induction coil for applying a magneticfield to said composite and heating said metal plates due to eddycurrents generated by the magnetic field; a power unit for applying analternating current to said induction coil to generate said magneticfield; and hot plates for heating both the top and bottom ends of saidcomposite.
 3. An apparatus for heating a rubber-metal plate compositecomprising a plurality of unvulcanized rubber layers and metal plates,each being overlaid alternately, by induction heating, said apparatuscomprising:an induction coil for applying a magnetic field to saidcomposite and heating said metal plates due to eddy currents generatedby the magnetic field; and a power unit for applying an alternatingcurrent to said induction coil to generate said magnetic field; and afrequency meter for changing and controlling the frequency of saidalternating current.
 4. An apparatus for manufacturing a vulcanizedrubber-metal plate composite comprising an apparatus for heatingdescribed in claim 1 and a compressing means for compressing saidcomposite in the longitudinal direction.